Monolithic integrated enhancement mode and depletion mode field effect transistors and method of making the same

ABSTRACT

A depletion mode (D-mode) field effect transistor (FET) is monolithically integrated with an enhancement mode (E-mode) FET in a multi-layer structure. The multi-layer structure includes a channel layer overlaid by a barrier layer overlaid by an ohmic contact layer. Source and drain contacts of the D-mode and E-mode FETs are coupled to the ohmic contact layer. A gate contact of the D-mode and E-mode FETs is coupled to the barrier layer. An amorphized region is provided beneath the E-mode gate contact within the barrier layer. The amorphized region forms a buried E-mode Schottky contact with the barrier layer. An alternative embodiment couples the gate contact of the D-mode transistor to a first layer that overlies the barrier layer, and provides a similar D-mode amorphized region within the first layer.

RELATED APPLICATION

The present application is a divisional of U.S. patent application Ser.No. 10/721,437 filed by Walter Anthony Wohlmuth on Nov. 24, 2003.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is in the field of integrated circuits, and inparticular involves field effect transistors.

2. Description of the Related Art

A type of semiconductor transistor known as a field effect transistor,or FET, includes three terminals: (1) a source; (2) a drain; and (3) agate. When a threshold voltage is applied to the gate, a “field effect”takes place in a region of semiconductor material under the gate, calledthe “gate region.” The effect is either a build up of charge or adepletion of charge in the gate region. Which event occurs depends onthe doping conductivity type of the gate region and the polarity of thegate voltage. The build up or depletion of charges creates a channelunder the gate that electrically connects the source and the drain. If achannel is present while the drain region is biased with a voltage, andthe source region is grounded relative to the drain region, then acurrent will flow through the channel between the drain and sourceregions.

Among the various types of FETs are enhancement mode (E-mode) anddepletion mode (D-mode) transistors. An E-mode transistor isnon-conductive when the gate voltage is zero or negative. For thisreason, an E-mode transistor is classified as a “normally off”transistor. An E-mode transistor is driven into conduction by bringingthe gate voltage positive with respect to the source voltage. In aD-mode transistor, by contrast, there is conduction even with zero gatevoltage, provided that the drain region is biased with a voltage, andthe source region is grounded relative to the drain region. For thisreason, D-mode transistors are classified as “normally-on” transistors.A D-mode transistor is made non-conductive by bringing the gate voltagenegative with respect to the source voltage.

One type of FET that is useful for high frequency applications is highelectron mobility transistor (HEMT). HEMT devices may be formed aseither enhancement mode or depletion mode devices, and often are formedfrom Group III-V materials, such as gallium arsenide (GaAs) and indiumphosphide (InP).

Generally, a HEMT includes a channel layer that is overlaid by a spacerlayer that is overlaid by a barrier layer. The spacer and barrier layersare formed of a wide band gap semiconductor material. The channel layeris formed of a narrow band gap semiconductor material. Due to theconduction band discontinuity at the junction between these dissimilarsemiconductor materials, which is called a “heterojunction,” electronsare injected from the barrier layer into the channel layer duringoperation of the transistor. The electrons are confined to move in aplane parallel to the heterojunction due to the relatively wider bandgapof the barrier layer. The electrons move more easily through the channellayer of a HEMT device than through the channel of an ordinary FET. As aresult, HEMT devices can operate at high speed and low noise levels.

In certain applications, it is desirable to form semiconductor devices,such as direct-coupled FET logic devices, that monolithically integratea D-mode transistor with an E-mode transistor in a single integratedcircuit. FIG. 1 discloses a conventional monolithically integratedD-mode/E-mode transistor device 1 that includes a D-mode transistor 2that is monolithically integrated with an E-mode transistor 3 in amulti-layer structure 5. In this particular example, each of the D-modeand E-mode transistors 2, 3 are a type of HEMT known as a pseudo-morphichigh-electron mobility transistor, or pHEMT. While the D-mode transistor2 and the E-mode transistor 3 are shown as being laterally adjacent toeach other for ease of view, the D-mode and E-mode transistors 2, 3 maybe disposed away from each other in different regions of the integratedcircuit.

Multi-layer structure 5 includes a semiconductor substrate 10 that isformed of undoped GaAs. Disposed in successive layers over semiconductorsubstrate 12 are various epitaxial semiconductor layers, including: abuffer layer 14; channel and spacer layers 16; an E-mode barrier layer18; an E-mode etch stop layer 20; a D-mode barrier layer 22; a D-modeetch stop 24; a wide recess transition layer 26; and an ohmic contactlayer 28. Note that there are two barrier layers and two etch stoplayers.

The D-mode transistor 2 and the E-mode transistor 3 each include a metalsource contact 38 and a metal drain contact 40 on an upper surface ofthe ohmic contact layer 28. Each transistor 2, 3 is electricallyisolated within a respective hollow column of implanted ions, depictedas isolation region 6, that surrounds the respective transistor 2, 3.The sidewall of the respective isolation region 6 around each transistor2, 3 extends downward from the upper surface of ohmic contact layer 28through the buffer layer 14.

Laterally between the source and drain terminals 38, 40 of both of theD-mode and E-mode transistors 2, 3 is a metal gate contact of therespective transistor 2, 3. The D-mode and E-mode gate contacts 30, 34are disposed in respective D-mode and E-mode gate recesses 32, 36 thatextend into multi-layer substrate 5 from the upper surface of ohmiccontact layer 90. The D-mode and E-mode gate contacts 30, 34 are coupledto different ones of the interior semiconductor layers of multi-layerstructure 5 at points vertically below the ohmic contact layer 90.

In particular, the D-mode gate contact 30 of D-mode transistor 2 iscoupled to an upper surface of D-mode barrier layer 22 within the D-modegate recess 32. The semiconductor sidewall 33 of the D-mode gate recess32 tapers inwardly going from the upper surface of ohmic contact layer28 downward toward D-mode barrier layer 22. In addition, the sidewall 33is stepped, so as to form a wide recess ledge 46 at the upper surface ofwide recess transition layer 26. The presence of the wide recess ledge46 is included to enhance the breakdown voltage of the D-mode transistor2.

Similarly, the metal gate contact 34 of E-mode transistor 3 is coupledto an upper surface of E-mode barrier layer 18 within the E-mode gaterecess 36. The semiconductor sidewall 37 of the E-mode gate recess 36also tapers inwardly going from the upper surface of ohmic contact layer28 downward toward the D-mode barrier layer 22. In addition, thesidewall 37 is stepped, so as to form a wide recess ledge 46 at theupper surface of wide recess transition layer 26. The presence of thewide recess ledge 46 is included to enhance the breakdown voltage of theE-mode transistor 3.

As shown in FIG. 1, the E-mode gate recess 36 and the E-mode gatecontact 34 extend vertically deeper into multi-layer structure 5 thanthe D-mode gate recess 32 and D-mode gate contact 30, because the E-modebarrier layer 28 to which the E-mode gate contact 34 is coupledvertically below the D-mode barrier layer 22 and the E-mode etch stoplayer 20.

To form the stepped D-mode gate recess 32, a plurality ofphotolithography and etch steps are required, including: (1) a firstselective etch step that etches the ohmic contact layer 28 through afirst photoresist mask and stops on wide recess transition layer 26; (2)a second selective etch step that etches the wide recess transitionlayer 26 through a second photoresist mask and stops on D-mode etch stoplayer 24; and (3) a third selective etch step that etches the D-modeetch stop layer 24 through the second photoresist mask and stops onD-mode barrier layer 22.

Likewise, a plurality of photolithography and etch steps are required toform the E-mode gate recess 36, including: (1) a first selective etchstep that etches the ohmic contact layer 28 through a first photoresistmask and stops on wide recess transition layer 26; (2) a secondselective etch step that etches wide recess transition layer 26 througha second photoresist mask and stops on D-mode etch stop layer 24; (3) athird selective etch step that etches D-mode etch stop layer 24 throughthe second photoresist mask and stops on D-mode barrier layer 22; (4) afourth selective etch step that etches the D-mode barrier layer 22through the second photoresist mask and stops on E-mode etch stop layer20; and (5) a fifth selective etch step that etches the E-mode etch stoplayer 20 through the second photoresist mask and stops on E-mode barrierlayer 18.

The gate contacts 30, 34 of the D-mode and E-mode transistors 2, 3,respectively, have the same structure, and consist of several successivemetal layers, including: (1) a thin first layer of titanium (Ti) incontact with the surface of barrier layers 22, 18, respectively; (2) athin second layer of platinum (Pt) on the Ti layer; and (3) arelatively-thick layer of gold (Au) on the Pt layer. The source anddrain contacts 38, 40 also are formed of several successive metallayers, including: (1) a first layer of Au in contact with the uppersurface of ohmic contact layer 28; (2) a second layer of nickel (Ni) onthe Au layer; (3) a third layer of germanium (Ge) on the Ni layer; and(4) a fourth layer of Au on the Ge layer.

The D-mode and E-mode gate recesses 32, 36 are typically filled with adielectric material around the metal gate contacts 30, 34. Thedielectric material may be silicon dioxide or silicon nitride. Metalinterconnects are coupled to the top surface of the gate contacts 30,34, the source contacts 38, and the drain contacts 40 to complete theD-mode and E-mode transistors 2, 3.

The conventional monolithic integrated E-mode/D-mode transistor device 1of FIG. 1 has several problematic aspects. First, the multi-layerstructure 5 includes a multiplicity of semiconductor layers, includingtwo etch stop layers 20, 24 and two barrier layers 18, 22. The greaterthe number of layers, the greater the cost and complexity of themulti-layer structure 5. This leads to additional cost and complexity inthe transistor fabrication process. For instance, the process forforming the E-mode gate recess 36 of FIG. 1 involves at least fiveselective etch steps.

In addition, the E-mode etch stop layer 20, which is necessary to themaking of E-mode transistor 3, is beneath the D-mode gate contact 30 andbarrier layer 22 of the D-mode transistor 2. Because manufacturingtolerances related to the thickness of the various epitaxial layers ofthe multi-layer structure 5 are up to 5% for a given epitaxial vendorand epitaxial growth machine, the structure of the multi-layer structure5 beneath D-mode transistor 2 is variable. Unfortunately, suchvariations in the placement of E-mode etch stop layer 20 will lead tovariability in the performance of the D-mode transistor 2 from wafer towafer. In addition, the presence of the E-mode etch stop layer 24beneath the D-mode gate contact 30 can also lead to other undesirableconditions, such as interfacial defects, interface roughness,non-uniformity, conduction and valence band offsets, and differentmaterial resistivities. These conditions also will degrade theperformance of the D-mode transistor 2.

The performance of the E-mode transistor 3 also is compromised in theconventional design. For instance, a certain degree of overetch isrequired to ensure that the E-mode etch stop layer 20 is reached duringthe step of etching through D-mode barrier layer 22. During theoveretching step, etching in the vertical direction proceeds slowlybecause of the selectivity of the etchant to E-mode etch stop layer 20.On the other hand, etching in the lateral direction through the D-modebarrier layer 22 proceeds unabated. As a result, the perimeter of thepartially-completed E-mode gate recess 36 at the exposed surface of theE-mode etch stop layer 20 is greater than desired. The subsequent etchstep that etches through E-mode etch stop layer 50 therefore exposes alarger-than-desired area of the upper surface of E-mode barrier layer18. Since the E-mode gate contact 34 only partially covers the exposedupper surface of the E-mode barrier layer 18 within E-mode gate recess36, an ungated region 44 on the surface of E-mode barrier layer 18 isformed. Control of the extent of the ungated region 44 is difficult dueto variations in the epitaxial layer thicknesses and etch dependencieson the feature size.

The relatively-large surface area of the semiconductor sidewall 37 andungated region 44 of the E-mode gate recess 36 is problematic. Thesesurfaces, which are covered by a native oxide due to exposure to theair, possess interface traps and defects due to the abrupt terminationof the regular crystal lattice, which in turn causes dangling bonds,defects, and surface states to form. The surface states will deplete theunderlying semiconductor material of charge carriers, and caneffectively cause the E-mode transistor 3 to be permanently offirrespective of the gate voltage applied to the E-mode gate contact 34.

Accordingly, a new approach to achieving a monolithically integratedD-mode/E-mode FET device is desirable.

SUMMARY

The present invention includes monolithically integrated D-mode/E-modeFET devices, and methods of making such devices.

In one embodiment, a monolithically integrated D-mode/E-mode FET deviceincludes a D-mode transistor and an E-mode transistor in a substrate.The substrate includes a plurality of semiconductor layers, including achannel layer overlaid by a barrier layer that is overlaid by an etchstop layer that is overlaid by an ohmic contact layer. Source and draincontacts of the D-mode and E-mode transistors are coupled to the ohmiccontact layer. Gate contacts of the D-mode and E-mode transistors arecoupled to an upper surface of the barrier layer. Beneath the E-modegate contact and within the barrier layer is an amorphized region thatincludes at least one layer of an electrically conductive compoundformed by a diffusion of a metal layer into the semiconductor of thebarrier layer. The amorphized region forms a Schottky contact withunderlying semiconductor portions of the barrier layer.

Accordingly, the monolithically integrated D-mode/E-mode FET device ofthis example that includes a single barrier layer, to which both theD-mode and E-mode gate contacts are coupled, and a single etch stoplayer. The structure of the multi-layer substrate therefore issimplified, rendering the making of the transistors easier and lessexpensive. Moreover, the performance of the D-mode and E-modetransistors is improved. The D-mode transistor is more reliable andrepeatable that in the conventional device, because the E-mode specificetch stop layer and barrier layer that were under the D-mode gatecontact and D-mode barrier layer of the conventional device areeliminated. The E-mode transistor performance is improved because theSchottky contact between the E-mode gate contact and the semiconductormaterial of the barrier layer is buried beneath the upper surface of thebarrier layer. This significantly reduces the risks posed by danglingbonds, defects, and surface states in the ungated region and gate recesssidewalls.

In an alternative embodiment, the D-mode transistor's gate contact isnot coupled to the same barrier layer to which the E-mode gate contactis coupled, but rather is coupled to an overlying semiconductor layer.The D-mode contact is, like the E-mode gate contact, formed in a mannerthat allows an initial metal layer of the D-mode gate contact to diffuseinto the semiconductor layer, thereby forming a D-mode amorphizationregion that provides a Schottky contact with underlying portions of thesemiconductor layer. This embodiment also improves on the conventionalstructure provided above, because it reduces the risks posed by danglingbonds, defects, and surface states in the ungated region and gate recesssidewalls.

These and other aspects of the present invention may be betterunderstood by reference to the following detailed description of theexemplary embodiments, when read in conjunction with the accompanyingdrawings thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified cross-sectional side view of a conventionalmonolithically integrated D-mode/E-mode FET device.

FIG. 2 is a simplified cross-sectional side view of an embodiment of amonolithically integrated D-mode/E-mode FET device in accordance withthe present invention.

FIGS. 3A-3L are cross-sectional side views of stages in a process formaking the monolithically integrated D-mode/E-mode FET device of FIG. 2.

FIG. 4 is a simplified cross-sectional side view of another embodimentof a monolithically integrated D-mode/E-mode FET device in accordancewith the present invention.

FIGS. 5A-5C are simplified cross-sectional side views of otherembodiments of a monolithically integrated D-mode/E-mode device inaccordance with the present invention.

FIG. 6 is a simplified cross-sectional side view of another embodimentof a monolithically integrated D-mode/E-mode FET device in accordancewith the present invention.

FIG. 7 is a simplified cross-sectional side view of another embodimentof a monolithically integrated D-mode/E-mode FET device in accordancewith the present invention.

Like features appearing in more than one figure are given the samereference numbers where possible, and redundant discussion thereoftypically is omitted.

DETAILED DESCRIPTION

The present invention includes an integrated circuit that includes bothD-mode and E-mode FETs, and a method of making the integrated circuit.The monolithically integrated D-mode and E-mode FETs formed inaccordance with the present invention may be, for instance,pseudo-morphic high-electron mobility transistors (pHEMTs),high-electron mobility transistors (HEMTs), modulation-doped fieldeffect transistors (MODFETs), meta-morphic high-electron mobilitytransistors (mHEMTs), heterojunction-insulated gate FETs (HIGFETs), andheterojunction field-effect transistors (HFET). Such devices may beformed, for instance, using a GaAs-based (gallium arsenide) materialsystem (GaAs, AlGaAs, InGaAs, AlAs, InGaAlAs, InGaP, InGaNP, AlGaSb,etc.), an InP-based (indium phospide) material system (InP, InAlP,InGaP, InGaAs, InAlAs, InSb, InAs, etc.), a Si and Ge-based (silicon andgermanium) material system (Si, Ge, SiGe, SiGeC, SiO2, SiC, sapphire,etc.), or a GaN-based (gallium nitride) material system (GaN, AlGaN,InGaN, InAlGaN, SiC, Si, sapphire, etc.), among other possibilities.

FIG. 2 illustrates a first embodiment of a monolithically integratedD-mode and E-mode FET device 101 in accordance with the presentinvention. In this particular example, the D-mode and E-mode transistors2, 103 of FIG. 2 are pHEMTs that are formed in a multi-layer structure105. While the D-mode transistor 2 is shown laterally adjacent to theE-mode transistor 103, the two transistors need not be laterallyadjacent, but rather may be disposed in different regions of the device101, which is a single integrated circuit. A respective isolation region6 isolates the D-mode transistor 2 and the E-mode transistor 103.

The monolithically integrated D-mode/E-mode FET device 101 of FIG. 2differs from the conventional monolithically integrated D-mode/E-modeFET device 1 of FIG. 1 by, for instance, including only a single etchstop layer 109 and a single barrier layer 107 within multi-layerstructure 105, rather than having the two etch stop layers 20, 24 andthe two barrier layers 18, 22 of the conventional structure of FIG. 1.Elimination of the E-mode etch stop layer 20 that formerly was beneaththe D-mode gate contact 30 and the D-mode barrier layer 22 (FIG. 1)resolves the aforementioned performance problems of the D-modetransistor 2 associated with varying layer thicknesses. Moreover, thecomplexity and cost of the multi-layer structure 105, and the number ofetch processes required to form the E-mode gate recess 36 of themonolithically integrated D-mode/E-mode FET device 101 of FIG. 2 aredecreased relative to those of the conventional device of FIG. 1.

The reduction in the number of semiconductor layers of the multi-layerstructure 105 of the monolithically integrated D-mode/E-mode FET device101 is achieved in part by using a different set of metal layers in theE-mode gate contact 111 of FIG. 2 than was used in the E-mode gatecontact 34 of FIG. 1. As is explained in greater detail below, theE-mode gate contact 111 is constructed such that the initial metal layerplaced in contact with the exposed upper surface of barrier layer 107within E-mode gate recess 110 diffuses into the semiconductor materialof barrier layer 107 in a controllable and uniform manner, forming anamorphized region 113. As the material of this initial metal layerdiffuses into barrier layer 107, a solid state interaction occurs toform small-grain-sized, uniformly stratified, electrically-conductive,elemental compound layers that form a Schottky contact with thesemiconductor material of barrier layer 107. This diffusion-assistedamorphization technique overcomes the aforementioned difficulties withthe ungated region 44 and E-mode recess semiconductor sidewall 37 ofE-mode transistor 3 of FIG. 1, since the interface between theamorphized region 113 and the barrier layer 107 is buried in themulti-layer substrate 105. Therefore, the detrimental surface effectsthat may be expected to arise from the ungated region 44 and the E-modegate recess sidewall 37 of the E-mode transistor 103 of FIG. 2 are muchreduced relative to the E-mode transistor 3 of FIG. 1, enabling a robustand repeatable E-mode transistor 103 to be realized. The control of thedepth of the amorphized material of amorphized region 113 is dependenton the type and thickness of the first-deposited metallic layer thatundergoes the amorphization, and on the process parameters used toinitiate, drive, and complete the reactions.

An exemplary method of making the monolithically integratedD-mode/E-mode FET device 101 of FIG. 2 begins with provision of anunpatterned wafer including multi-layer structure 105. The multi-layerstructure 105 includes a semiconductor substrate overlaid by a pluralityof epitaxial semiconductor layers. The multi-layer structure 105 may beformed of semiconductor materials within the various material systemsmentioned above. The number and types of epitaxial layers may vary. Theepitaxial layers may be grown using conventional molecular beam epitaxy(MBE) and metallorganic chemical vapor deposition (MOCVD) methods.

For the sake of example, we will discuss a multi-layer substrate 105that is within the GaAs-based material system. In one embodiment, thesemiconductor substrate 12 of multi-layer structure 105 is composed ofundoped GaAs. The buffer layer 14 is grown on the upper surface of thesubstrate 12. The buffer layer 14 may be composed of an initial bulkGaAs material layer on top of the substrate 12, followed by an optionalsuperlattice of alternating GaAs and Al_((x))Ga_((1-x))As materiallayers. Within the buffer layer 14, the bulk GaAs layer may have athickness (i.e., a vertical height in FIG. 2) that ranges between 100 to10,000 Å. The respective GaAs layers within the superlattice may have athickness ranging between 15 to 500 Å. The respectiveAl_((x))Ga_((1-x))As layers within the superlattice also may have athickness ranging between 15 to 500 Å. The Al_((x))Ga_((1-x))As layerscan have an (x) range from 0.0 to 0.5. Within the buffer layer 14, thealternating GaAs and Al_((x))Ga_((1-x))As layers may be repeated 5 to 50times. All of the layers in the buffer layer 14 are typically eitherunintentionally doped, slightly p-type doped, or slightly n-type doped.The superlattice typically, but not necessarily, terminates on aAl_((x))Ga_((1-x))As material layer.

Upon completion of the buffer layer 14, the channel and spacer layers 16are grown. The channel is composed of In_((y))Ga_((1-y))As, where (y)typically ranges between 0.10 to 0.50. The thickness of the channeltypically ranges between 50-200 Å. The channel layer is typicallyundoped and has a uniform composition. Other embodiments can include anon-uniform composition within the channel, such as a graded or steppedvariation in the indium content, and intentional doping within thechannel. The channel layer is formed of a narrow band gap semiconductormaterial. A spacer layer is disposed on either side of the channellayer.

The spacer layers are typically 15-60 Å thick and are composed ofAl_((x))Ga_((1-x))As, with (x) ranging from 0.0 to 0.5. The spacerlayers typically are undoped. In certain embodiments,In_((z))Ga_((1-z))P material may be used for the spacer layers, with (z)ranging from 0.4 to 0.6. The spacer layers are composed of materialsthat produce a large band offset relative to the In_((y))Ga_((1-y))Aschannel layer. The offset aids in the confinement of charge that istransferred into the channel layer. Within GaAs-based devices, thecharge carriers to be transferred into the channel layer preferably arecomposed of electrons rather than holes. Electron mobility and velocitywithin the channel layer is much greater than hole mobility andvelocity. Improvements in mobility and velocity are reflected byimprovements in the transistor's maximum frequency of operation andgain. The presence of dopants within the channel and/or the spacerlayers is generally avoided since these impurities degrade chargecarrier mobility and velocity by presenting scattering sites. Thescattering of charge carriers due to the presence of dopants reducescharge carrier mobility and velocity due to energy transfer and loss.

In one embodiment, silicon dopant layers, which may be one monolayer to60 Å thick, typically are grown on one or both sides of the channellayer at the spacer layer interface opposite to the channel—spacer layerinterface. The total dopant sheet carrier concentration is typicallybetween 0.0 and 6.0E12 cm². As the growth of the epitaxial layerstructure proceeds, charge from these dopant layers is transferred tothe channel layer.

Upon completion of the channel and spacer layers 16, including the uppersilicon dopant layer, a barrier layer 107 is grown. The barrier layer107 is a wide band gap semiconductor material, and may be composed ofAl_((x))Ga_((1-x))As, which can be undoped or doped with a doping levelranging from 0.0 to 1E18 cm⁻³. The (x) of the Al_((x))Ga_((1-x))As mayrange between 0.0 to 0.80. A thickness of the Al_((x))Ga_((1-x))As mayrange between 15 to 1500 Å. A thin layer of GaAs may be incorporated ontop of the Al_((x))Ga_((1-x))As layer to prevent surface oxidation ofhigh-aluminum-content Al_((x))Ga_((1-x))As. In certain embodiments, anIn_((z))Ga_((1-z))P material may be used for the barrier layer 107, with(z) ranging from 0.4 to 0.6, and a thickness ranging from 15 to 1500 Å.

An etch stop layer 109 is disposed on the upper surface of the barrierlayer 107. The etch stop layer 109 may be In_((z))Ga_((1-z))P, with the(z) ranging between 0.4 to 0.6. This layer may be undoped or doped witha doping level ranging from 0.0 to 1E18 cm⁻³. The In_((z))Ga_((1-z))Pmaterial can be ordered or disordered. The thickness of theIn_((z))Ga_((1-z))P layer may range from 10 to 100 Å.

In embodiments where the barrier layer 107 is an In_((z))Ga_((1-z))Player, then the etch stop layer 109 may be an Al_((x))Ga_((1-x))Aslayer, with the (x) ranging between 0.0 to 0.80, and a thickness rangingfrom 10 to 100 Å.

A wide recess transition layer 26 is grown on the upper surface of theetch stop layer 109. The wide recess transition layer may be composed ofAl_((x))Ga_((1-x))As, with (x) ranging from 0.0 to 0.80. The thicknessof this layer may range from 50 to 500 Å.

The ohmic contact layer 28 concludes the epitaxial layer stack ofmulti-layer structure 105. The ohmic contact layer may be anIn_((y))Ga_((1-y))As layer, where (y) ranges between 0.0 and 1.0. Thethickness of this layer may range from 50 to 1000 Å. The doping level inthis layer typically is as high as possible and is limited by the solidsolubility of the dopant in this layer. A dopant concentration may rangefrom 1E17 to 1E20 cm⁻³.

After the provision the multi-layer structure 105, the monolithicallyintegrated D-mode transistor 2 and E-mode transistor 103 are formed onmulti-layer structure 105. Steps in an exemplary method of making theD-mode transistor 2 and the E-mode transistor 103 include: (1) formingthe source and drain contacts 38, 40 on ohmic contact layer 28; (2)forming the D-mode gate recess 32 and then the D-mode gate contact 30 ofthe D-mode transistor 2; (3) forming the E-mode gate recess 110 and thenthe E-mode gate contact 111 of the E-mode transistor 103; (4) formingthe respective isolation regions 6 that surround the D-mode transistor 2and the E-mode transistor 103; (5) filling the D-mode and E-mode gaterecesses 32, 110 around the D-mode and E-mode gate contacts 30, 111 witha dielectric material; and (6) forming interconnects to the source anddrain contacts 38, 40 and to the D-mode and E-mode gate contacts 30,111. The particular order of these tasks may vary and some of thesesteps may be combined. Wet and/or dry etch methods are used for the etchsteps.

In an exemplary process, the ohmic contact layer 90 of multi-layersubstrate 105 preliminarily is overlaid with a blanket first dielectriclayer 301, as shown in FIG. 3A. The dielectric layer 301 may be a layerof silicon dioxide (SiO₂) or silicon nitride (Si₂N₃) deposited using aplasma-enhanced chemical vapor deposition (PECVD) process.

A first photoresist mask 303 then is formed over the first dielectriclayer 301 using conventional lithography tools and photoresist/developermaterials. As shown in FIG. 3B, the first photoresist mask 303 includesrelatively large-perimeter openings 305 for forming the upper,large-perimeter portion of both the D-mode gate recess 32 and the E-modegate recess 110. A first selective etch step etches the first dielectriclayer 301 through the openings 305 in the first photoresist mask 303,and stops on ohmic contact layer 28. A second selective etch step thenetches the ohmic contact layer 28 through the first photoresist mask 303and stops on wide recess transition layer 26, thus forming the upper,large-perimeter portions of the D-mode and E-mode gate recesses 32, 110.Subsequently, the first photoresist mask 303 and the first dielectriclayer 301 are stripped, resulting in the structure of FIG. 3C.

A blanket second dielectric layer 307, which may be silicon dioxide(SiO₂) or silicon nitride (Si₂N₃) deposited using a PECVD process, isthen deposited on ohmic contact layer 26. As shown in FIG. 3D, thesecond dielectric layer 307 blankets the upper surface of themulti-layer substrate 5, thereby filling the partially-formed D-mode andE-mode gate recesses 32, 110 and covering the exposed upper surface ofwide recess transition layer 26. Subsequently, a second photoresist mask309 is formed on the second dielectric layer 307. The second photoresistmask 309 includes openings 311 through which ions may be implanted usingconventional ion implantation equipment and techniques. The implantationstep forms isolation regions 6 that enclose the respective regions wherethe D-mode and E-mode transistors 2, 103 will be formed. Alternatively,the D-mode and E-mode transistors 2, 103 may be isolated usingconventional deep trench etch isolation methods.

Subsequently, the second photoresist mask 309 is stripped, and a thirdphotoresist mask 313 is formed over the second dielectric layer 307, asis shown in FIG. 3E. The third photoresist mask 313 includes openings315 through which metal layers may be deposited to form the source anddrain contacts 38, 40. The source and drain contacts 38, 40 includeseveral successive metal layers, including: (1) a first layer of gold(Au) in contact with the upper surface of ohmic contact layer 28; (2) asecond layer of nickel (Ni) on the Au layer; (3) a third layer ofgermanium (Ge) on the Ni layer; and (4) a fourth layer of Au on the Gelayer. A high temperature annealing process follows, which causes themetal layers of the source and drain contacts 38, 40 to intermix withthe semiconductor material of ohmic contact layer 28, as is shown inFIG. 3F.

Subsequently, the third photoresist mask 313 is stripped, and a processfor further forming the D-mode gate recess 2 is initiated. A fourthphotoresist mask 317 is formed over the second dielectric layer 307, asis shown in FIG. 3G. The fourth photoresist mask 317 includes an opening319 for forming the smaller-perimeter lower portion of D-mode gaterecess 32. Recalling that the second dielectric layer 307 was previouslydeposited within the upper portion of the D-mode gate recess 32, a firstselective etch step etches the second dielectric layer 307 through theopening 319 in the fourth photoresist mask 317 and re-exposes the uppersurface of the wide recess transition layer 26. A second selective etchstep etches the wide recess transition layer 26 through the fourthphotoresist mask 317 and stops on the etch stop layer 109. With thesecond selective etch step, the wide recess ledge 46 is formed. The widerecess ledge 46 remains covered by a portion of the second dielectriclayer 307. A third selective etch step then etches the etch stop layer109 through the fourth photoresist mask 317 and stops on barrier layer107. Accordingly, an upper surface of barrier layer 107 is exposed at abottom of the tapered, stepped D-mode gate recess 32.

The D-mode gate contact 30 is formed on the exposed upper surface of thebarrier layer 107 at the bottom of D-mode gate recess 32. The D-modecontact 30 includes a thin first layer of titanium that is deposited onthe exposed surface of barrier layer 107 though the opening 319 in thefourth photoresist mask 317. Various other electrically conductivematerials may be deposited over the initial titanium layer to completethe D-mode gate contact 30, including: tungsten (W), tungsten-silicide(WSi), gold (Au), platinum (Pd), palladium (Pd), molybdenum (Mo),iridium (Ir), tantalum (Ta), tantalum nitride (TaN), aluminum (Al), andhybrid combinations of these materials. Further titanium layers also maybe included in the stack of metal layers that form D-mode gate contact30.

For example, as is shown in FIG. 3H, an exemplary D-mode gate contact 30may include an initial titanium layer 320 that is deposited on theexposed upper surface of barrier layer 107 to a thickness of 50 to 1000Å. A layer of platinum 321 having a thickness of 50 to 1000 Å is thendeposited on the titanium layer 320. Finally, a gold layer 323 having athickness of about 500 to 10,000 Å is deposited on the platinum layer321.

The ungated region 44 of the D-mode transistor 2 may be sizeddifferently than the ungated region 44 of the E-mode transistor 103 inorder to improve breakdown characteristics.

After the D-mode gate recess 32 and D-mode gate contact 30 are formed,the fourth photoresist mask 317 is stripped, and a process for furtherforming the E-mode gate recess 110 is initiated. Referring to FIG. 3I, afifth photoresist mask 325 is formed over the second dielectric layer307. The fifth photoresist mask 325 includes an opening 327 for formingthe smaller-perimeter lower portion of E-mode gate recess 110. Recallingthat the second dielectric layer 307 was previously deposited within theupper portion of the E-mode gate recess 110, a first selective etch stepetches the second dielectric layer 307 through the opening 327 in thefifth photoresist mask 325 and re-exposes the upper surface of the widerecess transition layer 26. A second selective etch step etches the widerecess transition layer 26 through the fifth photoresist mask 325 andstops on the etch stop layer 109. With the second selective etch step,the wide recess ledge 46 is formed. The wide recess ledge 46 remainscovered by a portion of the second dielectric layer 307. A thirdselective etch step then etches the etch stop layer 109 through thefifth photoresist mask 325 and stops on barrier layer 107. Accordingly,as is shown in FIG. 3J, an upper surface of barrier layer 107 is exposedat a bottom of the D-mode gate recess 110. Note that the process offorming the E-mode gate recess 110 requires fewer etch steps than theprocess described above for forming the E-mode gate recess 36 of FIG. 1,because multi-layer structure 105 of FIG. 2 includes fewer layers thanmulti-layer structure 5 of FIG. 1

Subsequently, the E-mode gate contact 111 is formed on the exposedsurface of the barrier layer 107 at the bottom of E-mode gate recess110. The stack of metal layers that comprise the E-mode gate contact 111are deposited into E-mode gate recess 110 through the opening 327 in thefifth photoresist mask 325.

The metal layers deposited to form the E-mode gate contact 111 differfrom those that were deposited to form the D-mode gate contact 30 ofFIGS. 1 and 2, and the E-mode gate contact 34 of FIG. 1. In particular,the initial metal layer deposited on the exposed surface of the barrierlayer 107 at the bottom of the E-mode gate recess 110 is selected from agroup of metals that will fully amorphize with the semiconductormaterial of barrier layer 107 under selected process conditions, so asto form the amorphized region 113 of the monolithically integratedD-mode and E-mode FET device 101 of FIG. 2. Metals that may be used asthe initial metal layer deposited on the exposed surface of the barrierlayer 107 in forming E-mode gate contact 111 include, for instance,iridium (Ir), palladium (Pd), platinum (Pt), nickel (Ni), cobalt (Co),chromium (Cr), ruthenium (Ru), osmium (Os), rodium (Ro), and rhenium(Re). The subsequently-deposited layers of material used to form theremainder the E-mode gate contact 111 may be selected fromelectrically-conductive materials such as: tungsten (W),tungsten-silicide (WSi), titanium (Ti), gold (Au), platinum (Pd),palladium (Pd), molybdenum (Mo), iridium (Ir), tantalum (Ta), tantalumnitride (TaN), aluminum (Al), and hybrid combinations of thesematerials.

The solid state amorphization of the initial metal layer of the E-modegate contact 111 into the semiconductor of barrier layer 107 (FIG. 2)involves the use of a thermal treatment in the 250 to 400 degree Celsiusrange. Careful control of the temperature, and time at which the deviceis held at this temperature, is required to ensure that the initialmetal layer diffuses uniformly, consistently, and fully into the barrierlayer 107, so as to form the fully amorphized region 113. This thermaltreatment may be effected through a dedicated heating step, or though asubsequent step in the normal course of forming the D-mode and E-modetransistors 2, 103, e.g., a plasma-assisted dielectric deposition stepthat fills the D-mode and E-mode gate recesses 32, 110 after the D-modeand E-mode gate contacts 30, 111 are formed. Ideally, the amorphizationof the initial platinum layer into the barrier layer 107 will produceuniform, stratified layers of electrically conductive compounds withsmall grain size and very little intermixing of the stratified layers inorder to create repeatable and manufactureable transistors.

Referring to FIG. 3K, where barrier layer 107 is a layer ofAl_((x))Ga_((1-x))As having a thickness of 15 to 1500 Å, an exemplarymethod to form the E-mode gate contact 111 of FIG. 2 includes depositingan initial layer of platinum 327 on the exposed surface of the barrierlayer 107. The platinum layer 327 may have a thickness of 5 to 500 Å. Atitanium layer 329 having a thickness of 50 to 1000 Å is then depositedon the initial platinum layer 327. A second platinum layer 331 having athickness of about 50 to 1000 Å is then deposited on the titanium layer329. Finally, a gold layer 333 having a thickness of 500 to 10,000 Å isdeposited on the second platinum layer. In other words, Pt—Ti—Pt—Au isthe deposition sequence starting from the first layer to the last layer.After the thermal treatment, the amorphized region 113 may extend to adepth of about 10 to 1000 Å from the upper surface of barrier layer 107.Accordingly, about 5 to 500 Å of unreacted Al_((x))Ga_((1-x))As of thebarrier layer 107 remains below the amorphized region 113. Thestratified layers of compounds forming amorphized region 113 may includelayers of PtAs_((x)), where x can be between 0.5 and 2.0, andPtGa_((y)), where y can be between 0.5 and 3.0.

Within the exemplary Pt—Ti—Pt—Au structure of the E-mode gate contact110 of FIG. 3K, the gold layer 333 tends to lower the gate resistance,which improves high frequency transistor performance. The secondplatinum layer 331 serves as a diffusion/intermixing barrier between thetitanium and the gold layers 329, 333. The titanium layer 329 preventsthe second platinum layer 331 from undergoing solid state amorphizationwith the semiconductor material of barrier layer 107.

In alternative embodiments, wherein the initial metal layer deposited onthe barrier layer 107 in the course of forming E-mode gate contact 11 isone of the other metals listed above, e.g., iridium, palladium, nickel,cobalt, chromium, ruthenium, osmium, rodium, and rhenium, then thestratified layers of electrically-conductive compounds formed inamorphized region 113 would include the particular metal that wasinitially deposited on the barrier layer 107 obviously, the materialselected for forming the barrier layer 107 also will determine whatcompounds are formed in amorphized region 113.

In an alternative embodiment, a plurality of amorphizable metal layersselected from the above-mentioned list of metals may be sequentiallydeposited on the barrier layer 107 as the first few layers of the E-modegate contact 111. For instance, with reference to FIG. 3K, after thedeposition of platinum layer 327 in contact with the exposed uppersurface of barrier layer 107, but before the deposition of titaniumlayer 329, a layer of palladium may be deposited on the upper surface ofplatinum layer 327. The titanium layer 329 then would be deposited onthe palladium layer. Through a thermal treatment, as described above,the platinum and palladium metal layers would fully amorphize into thesemiconductor to form amorphized region 113, whereas the titanium layer329 would not diffuse into the barrier layer 107. Through selection ofthe plural amorphizable metal layers and the thicknesses thereof, theperformance of the E-mode transistor may be tuned for particularapplications.

Subsequent to the formation of the E-mode gate contact 110, the fifthphotoresist mask 325 is removed. Subsequently, a blanket dielectriclayer 335 is deposited over the multi-layer structure 105, as is shownin FIG. 3L. The dielectric layer fills the D-mode gate recess 32 aroundD-mode gate contact 30, fills the E-mode gate recess 110 around E-modegate contact 111, and covers the source and drain contacts 38, 40. Forinstance, the blanket dielectric layer 335 may be a layer of siliconnitride deposited using a PECVD process in the 250 to 400 degree Celsiusrange. As mentioned, the diffusion of the initial metal layer of theE-mode gate contact 111 into barrier layer 107 may occur during such adeposition step, thereby forming amorphized region 113. In some cases,small voids may form at the bottom of the D-mode and E-mode recesses 32,110 in the ungated region 44 around the D-mode and E-mode gate contacts30, 111 during the deposition of the dielectric layer 335.

Subsequent steps in the formation of the monolithically integratedD-mode and E-mode transistors 2, 103 include forming interconnects tothe source and drain contacts 38, 40 and to the D-mode and E-mode gatecontacts 30, 111 through the blanket dielectric layer 335. Conventionalphotolithography, etching, and metal deposition steps are used.

FIG. 4 illustrates another embodiment of a monolithically integratedD-mode and E-mode FET device 401 in accordance with the presentinvention. The monolithically integrated D-mode/E-mode FET device 401 isvery similar to device 101 of FIG. 2, and includes many of the samereference numbers.

In the monolithically integrated D-mode/E-mode FET device 401 of FIG. 4,the D-mode gate contact 430 of the D-mode transistor 402 is formed in amanner similar to the E-mode gate contact 111 of device 101 of FIG. 2,in that material deposited to form the D-mode gate contact 430 isdiffused into the upper surface of wide recess transition layer 26 ofmulti-layer substrate 105, forming a D-mode amorphization region 413beneath the D-mode gate contact 430. The D-mode amorphization region 413may have a lowermost portion that is entirely within wide recesstransition layer 26 (as is shown by a solid line), or may have alowermost portion that extends into the underlying etch stop layer 109(as is shown by a dash line). The depth of the D-mode amorphizationregion 413, which is controlled in large part by the thickness of theinitial layer of metal deposited to form the D-mode gate contact 430, isselected to achieve desired transistor performance. In a possiblealternative embodiment, the amorphization region 413 may extend tobarrier layer 107.

The monolithically integrated D-mode/E-mode FET device 401 of FIG. 4 maybe formed by slightly modifying the process described above for makingdevice 101 of FIG. 2. Referring to FIGS. 3F and 3G, the fourthphotoresist mask 317 is deposited over the second dielectric layer 307.A first selective etch step etches the second dielectric layer 307through the opening 319 in the fourth photoresist mask 317, therebyforming the D-mode gate recess 432 and exposing the upper surface of thewide recess transition layer 26. However, the subsequent etch steps thatproduce the structure of FIG. 3G are omitted. The D-mode gate contact430 is then formed within D-mode gate recess 432 on the exposed uppersurface of wide recess transition layer 26 by the deposition ofsuccessive metal layers through opening 319 in the fourth photoresistmask 317.

Similar to E-mode gate contact 111 of FIGS. 2 and 3K, the D-mode gatecontact 430 of FIG. 4 may be formed by depositing an initial metal layer(or layers) on the exposed surface of the wide recess transition layer26 that is (or are) selected from a group of metals that will fullyamorphize into the semiconductor material of wide recess transitionlayer 26 (and optionally into etch stop layer 65) under selected processconditions, so as to form the D-mode amorphized region 413 of the D-modetransistor 402. Metals that may be used as the initial metal layer(s)deposited on the exposed surface of the wide recess transition layer 26in forming the D-mode gate contact 430 include, for instance, iridium(Ir), palladium (Pd), platinum (Pt), nickel (Ni), cobalt (Co), chromium(Cr), ruthenium (Ru), osmium (Os), rodium (Ro), and rhenium (Re). Adiffusion/intermixing barrier layer such as titanium is deposited overthe to-be-amorphized initial metal layer(s). The subsequently-depositedlayers of material used to form the remainder the E-mode gate contact111 may be selected from electrically conductive materials such as:tungsten (W), tungsten-silicide (WSi), titanium (Ti), gold (Au),platinum (Pd), palladium (Pd), molybdenum (Mo), iridium (Ir), tantalum(Ta), tantalum nitride (TaN), aluminum (Al), and hybrid combinations ofthese materials.

As with the method for forming the E-mode gate contact 111, the solidstate amorphization of the initial metal layer of the D-mode gatecontact 430 into the semiconductor of wide recess transition layer 26involves the use of a thermal treatment in the 250 to 400 degree Celsiusrange. Such a thermal treatment may be accomplished though a dedicatedheating step, or though subsequent processes in the course making of thetransistors. For instance, the thermal treatment may be effected duringthe course of a PECVD process that deposits a silicon nitride layer tofill the D-mode and E-mode gate recesses 432, 110. As stated above, careshould be taken so that the initial metal layer deposited in the courseof making the D-mode gate contact 430 becomes fully amorphized into thesemiconductor. The D-mode and E-mode amorphized regions 413, 113 may beformed simultaneously in a single thermal treatment, or one may beformed before the metal of the other gate contact is deposited.

In an embodiment where the etch stop layer 109 and the wide recesstransition layers are Al_((x))Ga_((1-x))As layers of slightly differentcompositions, as discussed above, then the D-mode gate contact 430 ofFIG. 4 may be formed by initially depositing a layer of platinum on theexposed surface of wide recess transition layer 26 at the bottom ofD-mode gate recess 432. This first platinum layer may have a thicknessof 5 to 500 Å. A titanium layer having a thickness of 50 to 1000 Å isthen deposited on the initial platinum layer. A second platinum layerhaving a thickness of 50 to 1000 Å is then deposited on the titaniumlayer. Finally, a gold layer having a thickness of 500 to 10,000 Å isdeposited on the second platinum layer. In other words, Pt—Ti—Pt—Au isthe deposition sequence starting from the first layer to the last layerof the D-mode gate contact 430. The diffusing of the initial platinumlayer of the D-mode gate contact 430 into the wide recess transitionlayer 26 (an optionally into etch stop layer 109) forms stratifiedlayers of electrically conductive compounds, such as layers ofPtAs_((x)), where x can be between 0.5 and 2.0, and PtGa_((y)), where ycan be between 0.5 and 3.0.

A feature of the monolithically integrated D-mode/E-mode FET device 401of FIG. 4 is that, for both the D-mode transistor 402 and the E-modetransistor 103, the interface between the amorphized regions 413, 113and the underlying semiconductor material of the multi-layer substrate405 is buried. Therefore, the detrimental surface effects that may beexpected to arise from the ungated region 44 and the D-mode and E-modegate recess sidewalls 33, 37 are much reduced, enabling robust andrepeatable D-mode and E-mode transistors 402, 103 to be realized.

FIGS. 5A-5C illustrate other monolithically integrated D-mode and E-modeFET devices 501A-501C, respectively, in accordance with the presentinvention. The monolithically integrated D-mode/E-mode FET devices501A-501C are very similar to device 101 of FIG. 2, and includes many ofthe same reference numbers. Our discussion can therefore focus on thedifferences between D-mode/E-mode FET devices 501A-501C andD-mode/E-mode FET device 101 of FIG. 2.

Referring to FIG. 5A, D-mode gate contact 30 is coupled to an exposedupper surface of etch stop layer 109, which is intact over barrier layer107, rather than to barrier layer 107 itself as in FIG. 1. Referring toFIG. 5B, E-mode gate contact 111 is coupled to an exposed upper surfaceof etch stop layer 109, which is intact over barrier layer 107, ratherthan to barrier layer 107 itself as in FIG. 1. Amorphization region 113extends through etch stop layer 109 and into barrier layer 107.Referring to FIG. 5C, both the D-mode gate contact 30 and the E-modegate contact 111 are coupled to an exposed upper surface of etch stoplayer 109, which is intact over barrier layer 107, rather than tobarrier layer 107 itself as in FIG. 1. Fabrication of the D-mode/E-modeFET devices 501A-501C therefore entails omitting the step of etchingthrough etch stop layer 109 in the course of forming the D-mode gaterecess 32 and/or the E-mode gate recess 110. Forming the gate contacts30, 111 on the etch stop layer 109 rather that the barrier layer 107will alter the electrical properties of the transistors, in ways thatmay be desirable for certain applications.

FIG. 6 illustrates another embodiment of a monolithically integratedD-mode and E-mode FET device 601 in accordance with the presentinvention. The monolithically integrated D-mode/E-mode FET device 601 isvery similar to device 101 of FIG. 2, and includes many of the samereference numbers. In device 601, an ion implant region 603 is formed inbarrier layer 107 after the formation of E-mode gate recess 110 butprior to the deposition of the initial metal layer of the E-mode gatecontact 111. For instance, if the barrier layer 107 in doped to beN-type, the implant region 603 is P-type. After the deposition of themetal layers of the E-mode gate contact 111, and after the thermaltreatment mentioned above, amorphization region 113 forms within (fullyor at least partially within) the tear-drop-shaped implant region 603.Such a design may have improved performance, e.g., with respect toleakage currents.

FIG. 7 illustrates another embodiment of a monolithically integratedD-mode and E-mode FET device 701 in accordance with the presentinvention. The monolithically integrated D-mode/E-mode FET device 401 issimilar to device 101 of FIG. 2, and includes many of the same referencenumbers. A difference between the embodiments is that the multi-layersubstrate 705 of device 701 includes fewer epitaxial semiconductorlayers than multi-layer substrate 105 of device 101. In particular, themulti-layer substrate 705 includes a semiconductor substrate 12,overlaid by a channel and spacer layer 16, overlaid by a barrier layer107. The spacer layer(s) and barrier layer 107 are formed of a wide bandgap semiconductor material. The channel layer is formed of a narrow bandgap semiconductor material.

The source and drain contacts 38, 40 of the D-mode transistor 702 andthe E-mode transistor 703 of device 701 formed on an upper surface ofbarrier layer 107. A high temperature annealing process follows, whichcauses the metal layers of the source and drain contacts 38, 40 tointermix with the underlying semiconductor material, thereby forming aintermixed region 707 that extends through channel and spacer layer 16.Alternatively, an ion implant step may be performed prior to thedeposition of the metal layers of source and drain contacts 38, 40, sothat an implantation region is present beneath the source and draincontacts 38, 40.

The D-mode gate contact 30 and the E-mode gate contact 111 also areformed on the upper surface of barrier layer 107. The E-mode gatecontact 111 is formed to have an initial metal layer (or layers) thatwill diffuse into barrier layer 107 upon a heat treatment, so as to forman amorphization region 113 within the barrier layer 107, therebyforming a buried Schottky contact. For instance, the E-mode gate contact111 may be formed with an initial platinum layer 328, which is overlaidby a titanium layer 329.

This disclosure provides exemplary embodiments of the present invention.The scope of the present invention is not limited by these exemplaryembodiments. Numerous variations, whether explicitly provided for by thespecification or implied by the specification, such as variations instructure, dimension, type of material and manufacturing process may beimplemented by practitioners in view of this disclosure.

1. A method of making an integrated circuit including a D-mode FET andan E-mode FET, the method comprising: providing a multi-layer structurecomprising a semiconductor substrate overlaid with a plurality ofepitaxial semiconductor layers, including a channel layer overlaid by abarrier layer overlaid by an etch stop layer, wherein the channel, etchstop and barrier layers are common to the D-mode and the E-mode FETs;forming respective source and drain contacts for the D-mode FET andE-mode FET on one of the epitaxial layers of the multi-layer structure;forming a gate recess in the multi-layer structure for the D-mode FET,and a gate recess in the multi-layer structure for the E-mode FET,wherein a surface of the barrier layer is exposed at a bottom of boththe D-mode and E-mode gate recesses; and depositing a plurality of metallayers onto the exposed surface of the barrier layer within the D-modeand E-mode gate recesses, thereby forming a D-mode gate contact and anE-mode gate contact on the barrier layer, wherein the E-mode gatecontact forms a Schottky contact with the barrier layer, wherein a firstmetal layer deposited in contact with the barrier layer in the D-modegate recess is different than a first metal layer deposited in contactwith the barrier layer in the E-mode gate recess.
 2. The method of claim1, further comprising: fully amorphizing the first metal layer depositedin contact with the barrier layer in the E-mode gate recess into thebarrier layer, wherein the first metal layer deposited in contact withthe exposed surface of the barrier layer in the D-mode gate recess isnot significantly amorphized into the barrier layer.
 3. The method ofclaim 2, wherein the first metal layer deposited in contact with theexposed surface of the barrier layer in the E-mode gate recess comprisesone of platinum, iridium, palladium, nickel, cobalt, chromium,ruthenium, osmium rodium, and rhenium, and the first metal layerdeposited in contact with the barrier layer in the D-mode gate recess isnot one of iridium, palladium, nickel, cobalt, chromium, ruthenium,osmium rodium, and rhenium.
 4. The method of claim 2, wherein thebarrier layer is of a first conductivity type; and further comprisingforming an implant region of a second conductivity type at least in thebarrier layer beneath the E-mode gate contact, wherein the first metallayer of the E-mode gate contact is amorphized into the implant region.5. The method of claim 1, wherein the first metal layer deposited incontact with the exposed surface of the barrier layer in the E-mode gaterecess comprises one of platinum, iridium, palladium, nickel, cobalt,chromium, ruthenium, osmium rodium, and rhenium; and further comprising:depositing a second metal layer on the E-mode first metal layer, whereinthe second metal layer is a different one of platinum, iridium,palladium, nickel, cobalt, chromium, ruthenium, osmium rodium, andrhenium than the first metal layer; and fully amorphizing second metallayer of the E-mode gate contact into the barrier layer.
 6. A method ofmaking an integrated circuit including a D-mode FET and an E-mode FET,the method comprising: providing a multi-layer structure comprising asemiconductor substrate overlaid with a plurality of epitaxialsemiconductor layers, including at least a channel layer overlaid by abarrier layer overlaid by an etch stop layer, wherein at least thechannel, etch stop and barrier layers are common to the D-mode and theE-mode FETs; forming respective source and drain contacts for the D-modeFET and E-mode FET on the multi-layer structure over the barrier layer;forming a gate recess in the multi-layer structure for the E-mode FET,wherein a surface of the barrier layer is exposed at a bottom of theE-mode gate recess; depositing a plurality of metal layers onto themulti-layer structure to form a D-mode gate contact on the barrierlayer; depositing a plurality of metal layers onto the exposed surfaceof the barrier layer within the E-mode gate recess, thereby forming anE-mode gate contact on the barrier layer, wherein the plurality of metallayers includes a first metal layer in contact with the exposed surfaceof the barrier layer, and wherein the E-mode gate contact forms aSchottky contact with the barrier layer; and fully amorphizing the firstmetal layer of the E-mode gate contact into the barrier layer, wherein afirst metal layer of the D-mode gate contact that was deposited incontact with the multi-layer structure is not significantly amorphizedinto the multi-layer structure.
 7. The method of claim 6, furthercomprising: forming a gate recess in the multi-layer structure for theD-mode FET, wherein a surface of the barrier layer is exposed at abottom of the D-mode gate recess, wherein the first metal of the D-modegate contact is in contact with the barrier layer.